1. Field of the Invention
The present invention relates to a method of recording information by irradiating an optical information recording medium such as an optical disk with an optical beam such as a laser beam and changing the optical characteristics of an optical disk portion irradiated with the optical beam, and an apparatus therefor.
2. Description of Related Art
An optical information recording medium such as an optical disk normally has a substrate and a recording layer formed on the substrate. When the recording layer is irradiated with an optical beam, the optical characteristics of the portion irradiated with the optical beam are changed and thus information is recorded. As a recording mechanism used for recording information in accordance with this method, the following four mechanisms are conventionally known:
(1) The recording layer is made of a metal which can be easily fused. The recording layer is irradiated with an optical beam, and its portion irradiated with the optical beam is fused by the heat of the optical beam to form pits. Information is thus recorded.
(2) The recording layer is made of a magnetic material. The recording layer is irradiated with an optical beam, and the direction of magnetization of the portion irradiated with the optical beam is inverted by an external magnetic field. Information is thus recorded.
(3) The recording layer is made of a material which can be phase-transited between, e.g., crystalline and amorphous phases. The recording layer is irradiated with an optical beam, and phase transition is caused at its portion irradiated with the optical beam. Information is thus recorded.
(4) The recording layer is made of an organic material. The recording layer is irradiated with an optical beam, and the molecular structure of its portion irradiated with the optical beam is changed. Information is thus recorded.
Among these, mechanism (1) is used most widely. When information is to be recorded in accordance with this mechanism, the apparatus shown in FIG. 1 is used. When information is to be recorded on optical disk 6 by the apparatus shown in FIG. 1, as disk 6 is rotated at a predetermined speed, a laser pulse is emitted from laser beam source 1 in accordance with the information to be recorded which is supplied from a signal processor (not shown). The laser pulse is converted into a collimated beam by lens 2. The collimated beam passes through polarizing beam splitter 3 and .lambda./4 plate 4, and focused by focusing lens 5 to irradiate recording layer 7 formed on substrate 6a of disk 6. Laser pulse irradiated portion 7a is fused to form a pit.
When information is to be read out, a continuous optical laser beam of a lower output power than that in the case of information recording is emitted from laser source 1. The laser beam reflected by recording layer 7 passes lens 5 and .lambda./4 plate 4, is reflected by beam splitter 3, and is focused by focusing lens 8 toward optical detector 9. The intensity of the reflected light is detected by detector 9, and an obtained detection signal is supplied to a signal processor (not shown) to reproduce the recorded information.
In the recording apparatus described above, when information to be recorded is digital information, the output timing of the laser pulses has a predetermined period. Whether a laser pulse is to be actually output or not is controlled in accordance with whether the digital information is binary "1" or "0". More specifically, portions of optical disk 6 corresponding to "1" are recording pits 7a having the substantially same shape, and those corresponding to "0" are non-pit portions 7b indicated by broken lines, as shown in FIG. 2. Assuming that a pit is formed even in each non-pit portion 7b, the pitches between the pits become the same.
When information to be recorded is analog information, the length and interval of the laser pulses to be output ar changed in accordance with the information. Pits 7a are formed such that both their shapes and pitches between adjacent pits are different, as shown in FIG. 3.
When an optical disk uses the recording mechanisms (2) to (4), recording apparatuses similar to that described above are used.
Recently, however, high-density recording must be performed on an information recording medium. In this case, with the conventional recording method described above, the recording density is sometimes insufficient.